Wireless audio signal transmission method for a three-dimensional sound system

ABSTRACT

A three-dimensional sound system includes a wireless audio signal transmission method for transmitting audio signals between a transmitting device and a spatially adjacent receiving device associated with an audio signal reproduction device such as a speaker. The audio signals may be digitized in the transmitting device, compressed, and transmitted by a digital high-frequency transmission method as data packets. Symbols may be assigned to the individual data in a quadrature signal plane. A diversity operation takes place between the transmitting device and the receiving device, where the transmitting device has two high-frequency transmitters with quadrature conversion which are each connected to an associated transmission antenna. On the receiver side, each audio signal reproduction device has a receiving device with one receiving antenna and one high-frequency receiver. Differentiation of the two received high-frequency data streams having the individual symbols in coded form may be implemented in a decoding device.

PRIORITY INFORMATION

This application claims priority from International Patent ApplicationNo. PCT/EP03/06816 filed Jun. 27, 2003, and German Patent ApplicationNo. 102 29 266.3 filed Jun. 28, 2002.

BACKGROUND OF THE INVENTION

The invention relates in general to audio reproduction systems and inparticular to a wireless audio signal transmission method for athree-dimensional sound system.

In the home setting, modern audio reproduction systems are increasinglyintended to provide multichannel sound reproduction based on the Dolbydigital standard, the Digital Theater Standard (DTS), or some otherthree-dimensional sound method, in combination with a televisionreceiver for digital reception or with a DVD player. With these systems,the audio signals are typically transmitted to up to six differentspeaker locations. In the home setting, however, the requiredinstallation of physical signal lines is often a problem. For thisreason, there is often a desire to have wireless transmission thatenables playback devices and speakers in different rooms to beinterconnected.

Known wireless solutions are based on transmission links using frequencymodulation. However, the quality of this type of analog transmission forspeakers or headphones usually does not meet more demandingrequirements. In addition, analog transmission is susceptible tointerference, is not secure against being intercepted, and isinefficient in utilizing the available bandwidth. In the home setting,disturbed reception conditions are also to be expected due toreflections and shadowing.

An improvement is to replace the analog signal transmission by thetransmission of data which have been generated by prior sampling anddigitization of the analog signals. An example of wireless digital audiosignal transmission is European patent application EP 0 082 905 A1.Using an infrared transmission device, digitized audio signals aretransmitted by a transmitting device (e.g., a television receiver) to“active speaker boxes” within the room. The inconvenient physical signallines are eliminated, while simple connections to the standard AC powersupply provide power. Unfortunately, while this system is suitable forstereo signals, it is not applicable to multichannel sound systemtechniques.

What is needed is a multichannel sound system that avoids theabove-described disadvantages without increasing the cost by anunreasonable amount.

SUMMARY OF THE INVENTION

In a wireless audio signal transmission method for a three-dimensionalsound system, the audio data for one or more audio signal transmittingdevices are digitized, and the digitized data are transmitted as symbolsby a digital modulation method. The number of required high-frequencychannels is typically determined by the bandwidth specified for eachchannel together with the total bandwidth of the frequency range used.This method of transmission using symbols may employ a diversity method.Specifically, the interference caused by multipath reception andshadowing may be reduced through use of a diversity method. Thepropagation of HF and UHF signals within spaces is typicallycharacterized by a plurality of mutually independent propagation pathsfrom the transmitter to the receiver. In addition to a relativelystrongly attenuated direct path, multiple indirect paths may arisedepending on whether or not obstacles are present. Since the resultingpath lengths typically differ, the individual audio signals generallyarrive at the receiver at different phase positions. When the phaseoffset is 0°, 360°, or a multiple thereof, this is known as constructiveinterference. If, on the other hand, the phase offset is 180°, or 180°plus a multiple of 360°, this is known as destructive interference. Ifthe two signals are equally strong, then the two signals cancel out eachother. This effect is dependent on frequency since the phase shift overa fixed path length is a function of frequency. For example, fieldstrength measurements between a transmitter and a receiver for which amovement occurred in an indoor space over a 15 meter path havingreflections and obstacles demonstrated field strength drops of up to 30dB at a frequency of 864 MHz, where the direct propagation path wasattenuated by an obstacle.

In modern FM wireless speakers, this situation may be avoided throughcareful placement of the receiver. Since, however, people must also betaken into account as obstacles or reflectors, their movement results ina change in propagation conditions. This occurs, for example, if thereceiver is portable, as with battery-powered headphones having awireless connection to the transmitting device and a correspondingreceiving device.

A simple solution may be to increase the transmission power. However,for legal reasons this is usually not possible with the availablefrequencies. Since the interference effects are a function of locationand path, a solution may be to implement two or more mutuallyindependent transmission paths using a diversity method. The frequencydependence of the interference phenomena can be exploited bytransmitting on two different frequencies simultaneously, then selectingthe better signal on the receiver side. However, this solution is noteconomical in terms of frequency. Another approach is receiverdiversity. To maintain the independent paths needed for propagation, tworeceiving antennas are set up at a distance of at least a wavelength ofλ/4 from each other. Either the relatively stronger antenna signal isselected by the receiver, or the two signals are combined. To avoiddrop-outs during switching, this approach requires, however, that atleast two receivers in complete form up to recovery of the channel-codeddata be provided at each receiving site.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art transmitter and receiverdiversity system having two separate channels;

FIG. 2 is a schematic diagram of a prior art transmitter diversitysystem with a single receiver;

FIG. 3 is a schematic diagram of a prior art receiver diversity systemwith a single transmitter;

FIG. 4 is a schematic diagram of a transmitter diversity system withseparate transmitting channels and a single receiving channel;

FIG. 5 is a table illustrating the transmission of different datasequences within the system of FIG. 4;

FIG. 6 is a block diagram of a transmitter portion of the system of FIG.4;

FIG. 7 is a block diagram of a receiver portion of the system of FIG. 4;and

FIG. 8 illustrates two different data formats that may be utilized bythe receiver of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

An advantage provided by digitization of audio signals to be transmittedis a higher level of immunity against interference due to quantizationwhich can then be further enhanced through the addition of check bits orother error-detection or error-correction methods. Another advantage isthat known methods of compression for data reduction involve redundantproperties of the audio signals to reduce the amount of data fortransmission without any appreciable loss in quality.

Unfortunately, the use of a diversity method increases the number ofaudio channels. For example, when using diversity methods, normally onetransmitter and one receiver are required for each audio channel, asillustrated by the prior art diversity system 10 of FIG. 1. If eachaudio channel is designed in duplicate form, the resulting requirementfor six speaker sites is twelve high-frequency (HF) audio channels, andan equal number of transmitters, receivers, and antennas. This approachis generally not cost-effective.

Referring to FIG. 1, a known diversity method implemented in a system 10with two audio channels includes a signal source 12 connected to areproduction device 14 (e.g., a speaker) through two transmitters 16, 18with two antennas 20, 22, and two receivers 24, 26 with two antennas 28,30. Two audio signals 32, 24 transmitted through the correspondingtransmitting antennas 20, 22 have different transmission frequencies f1,f2. Evaluation of the received signals 32, 34 and generation of theactual audio signals therefrom for output through the speaker 14 may beimplemented in an attached electronics system 36.

In the system 10 of FIG. 1, diversity is achieved based on thefrequency-dependent propagation conditions for the two transmissionfrequencies f1, f2. This is because the phase positions due toreflections and obstacles vary, and generally given differentfrequencies an attenuation or even a cancellation may occur. The resultis that one of the received signals 32, 34 typically has sufficientfield strength. Additional improvements are possible by making thespacing between the transmitting antennas 20, 22 or the receivingantennas 28, 30 as large as possible, or by making the polarity andemission direction or reception direction different relative to eachother. These measures can be carried out singly or in combination. Afurther improvement can be achieved not only by having the two receivers24, 26 each detect one of the two different transmission frequencies f1,f2, but also by designing them to be as broadband as possible such thatboth frequencies f1, f2 are received. Separation of the frequencies andtheir contents may be carried out internally by filtering. The number oftransmission paths is then doubled so that any undesired cancellationsare less likely to occur.

A more simplified approach may be provided by known, one-sided diversitymethods which have separate transmission channels or receiving channelseither on the transmitter side (FIG. 2), or on the receiver side (FIG.3), opposite which channels is a single receiver 38 (FIG. 2) or a singletransmitter 40 (FIG. 3). In the known transmitter diversity methodillustrated in the system 42 of FIG. 2, the audio signals 32, 24 aretransmitted by the two transmitters 16, 18 and the corresponding twoantennas 20, 22, on two different frequencies f1, f2. On the receiverside, the two audio signals 32, 34 passing along different propagationpaths are superimposed on each other and are detected by a singleantenna 44 with the associated receiver 38. The transit time differencesdue to the frequency diversity and space diversity generally prevent anysimultaneous total cancellation of the two frequencies f1, f2. In thereceiver 38, either the signal content of the two frequencies f1, f2 isheterodyned, or that frequency is selected which at that instant has thehigher field strength. An alternative system (not shown) of thatillustrated in FIG. 2 employs the same transmitting antenna for bothfrequencies f1, f2. In this case, frequency diversity exists.

In the known receiver diversity method illustrated in the system 46 ofFIG. 3, a single transmitter 40 transmits a signal 48 at thetransmission frequency f through an antenna 50. On the receiver side,the signal 48 is received by the two separate antennas 28, 30 and theassociated receivers 24, 26 to which, as in FIG. 1, the commonelectronics system 36 is attached which ultimately feeds thereproduction device 14. This method involves space diversity, althoughdirectional diversity or polarity diversity can be added through thereceiving antennas 28, 30. The two signals from the receivers 24, 26 mayeither be heterodyned in the attached electronics system 36, or thesystem 36 may have a selection circuit which further processes theantenna signal with the higher field strength.

Receiver diversity may commonly be employed, for example, inprofessional settings for portable microphones since this type ofsituation ordinarily does not allow for multiple transmitting antennas.The frequency-modulated signal from the microphone transmitter isreceived by the associated receiver which is coupled to two extendableantennas, each of which is attached to a high-frequency receiver. Whilethe diversity method may not be advantageous in this situation due tothe relatively close spacing of the receiving antennas, the cost andcomplexity of the electronics involving sensitive receivers and thefurther relaying and processing of the signals are not of relativelyhigh importance. If necessary, an additional receiver may be utilized.

For applications in the home setting, multiple antennas located inspeakers may not be desirable for aesthetic reasons. Thus, the diversitymethods in the systems 10, 46 of FIG. 1 and FIG. 3, respectively, aregenerally not utilized. However, a modified transmitter diversity methodthat is a refinement of the system 42 of FIG. 2 may be utilized, whichis typically capable of transmitting data sequences. Any added expensein terms of equipment is essentially on the transmitter side and not thereceiver side.

FIG. 4 illustrates a transmitter diversity system 60 with two separatetransmitting channels and a single receiving channel. FIG. 5 is a tableillustrating how the transmission of two different data sequences isimplemented in the system 60 of FIG. 4 using the same transmissionfrequencies, as in the known space-time block code method. Theprinciples of this method are described in detail for differentvariants, for example, in IEEE Signal Processing Magazine, May 2000,pages 76 to 91, in the article “Increasing Data Rate over WirelessChannels” by Ayman F. Naguib, Nambi Seshadri, and A. R. Calderbank. Touse this method in the system 60 with high-end audio reproductiondevices 62, a signal source 64 may supply data as audio signals, or, inthe case of analog signals, digitization may occur in the source 64 orin an attached encoder 66.

On the transmitting side 68 of the system 60 of FIG. 4, a data streamD_(o) on a line 70 to be transmitted is processed within the encoder 66and provided as first and second data streams D₁, D₂, on lines 72, 74,respectively, to a transmitter stage 76 having high-frequencytransmitters 78, 80. The data streams D₁, D₂ are then transmittedthrough two spatially separated antennas 82, 84 as quadrature-modulatedsignals 86, 88, but in the same frequency band f despite differentcontents.

On the receiving side 90 of the system 60 of FIG. 4, a single antenna 92along with a high-frequency receiving device 94 and a decoder 96 recoverthe original data sequence D_(o) from the heterodyned signals r on aline 98 from the antenna 92 or from a data sequence Dr on a line 100from the receiving device 94 generated therefrom. The data sequence maybe further processed and reproduced in the audio reproduction device 62.

Data compression on the transmitter side 68 may be utilized.High-frequency channels are relatively narrow-band and have a typicalchannel width of, for example, 300 kHz. By using data compression, it ispossible to transmit data from two or more audio channels on onehigh-frequency channel. Data compression may exploit the redundancy inthe audio signals, the right and left channel information of symmetricalspeaker locations being suitable for this type of compression. The datastream may then be converted into symbols that are transmitted by thehigh-frequency carrier.

The digital transmission of symbols requires on the receiver side 90 anevaluation of the received signal at predefined times at which thetransmitted signal occupies a defined state in the quadrature signalplane. To determine the state that corresponds to the transmittedsymbol, the received signal is sampled and digitized, at least atdefined times. The reduction of any interference, subsequent conversion,and decoding may also be implemented digitally. In zero-IF or low-IFreceivers in which the two quadrature components are converted directlyto the baseband or a low frequency position where they are digitized,receiving concepts can be provided that can be embodied within a singleIC for each receiver, without significant external circuit elements.After frequency conversion, the decoding and subsequent signalprocessing may be implemented in a single digital signal processor.Thus, any inaccuracies in the analog component, such as phase errors oramplitude errors, can be corrected in the processor since asymmetriesand inaccuracies as separate error sources are generally not possible.

In selecting a transmission band, a number of suitable high-frequencybands are available. The approved frequency range between 433.020 MHzand 434.790 MHz, also known as the “ISM band,” is less well suited sincein this range there is no protection from other users or from thepriority-status transmissions of amateur radio. Not only would an alarmsystem or a wirelessly-controlled central locking system of anautomobile interfere, the FM signal can be intercepted. The 863 MHz to865 MHz frequency band reserved for audio transmission has found onlyreluctant acceptance, likely because the 10 mW approved radiated power(ERP) is relatively low for operation not subject to individualcertification. Within close range, the use of this frequency band forthe wireless control of audio reproduction devices may be suitable ifthe transmitting and receiving antennas are within sight of each other.Otherwise degradations in reception may result. As mentionedhereinabove, the transmitted audio signal is not only subject toattenuation but also to multiple reflections. Whenever two of thesesignal components arrive at the receiver in phase opposition but withapproximately the same intensity, they cancel each other completely. Inthe extreme case, an almost complete loss of reception may result.

A frequency band around 40 MHz is not suitable due to the narrowbandwidth. Strong interference may occur in the segment around 432 MHzin the 70-cm amateur band. Frequencies in the GHz range are not suitablebased on the higher component costs and increasingly unfavorablepropagation conditions. In addition, the lowest portion of this rangearound 2450 MHz is already utilized by a number of services and userssuch as Bluetooth, wireless data links, and microwave ovens. Whatremains is thus the range around 864 MHz. This range is specificallyintended for wireless audio applications in streaming mode (dutycycle=1), that is, the high-frequency carrier in each channel can be inaction continuously. Due to the limited bandwidth of only 2 MHz for thisentire frequency band, the audio data have to be compressed. To providesimultaneous video reproduction, lip-synchronicity is required, with theresult that allowable delay between video and sound is approximately 20ms This delay is relevant in light of the chosen compression methodalong with the desire for highest possible fidelity of reproduction.Compression methods that computationally compress the 16-bit or 24-bitaudio data to six bits per sampling value are known. For example, seethe adaptive differential pulse code modulation (ADPCM) method or othermethods in K. D. Kammeyer, “Information Transmission”, B. G. TeubnerStuttgart, 2^(nd) edition 1996, pages 124 through 137, Chapter 4.3entitled “Differential Pulse Code Modulation.” A stereo signal sampledat 48 kHz yields a data rate of 576 kB/s. Higher-level compressionmethods such as MP3 that enable a stronger compression are not suitablesince their delay is too large. Also, a transmitter-side preliminarydelay of the video information in the home setting is too complex.

The 16-QAM method may be selected as the digital modulation approach totransmit the symbols. This method represents a compromise betweentransmission capacity and implementability. Extensive system analysesshow that a ¾ trellis coding of the modulation provides for sufficienterror protection. The gross data rate for the stereo signal isapproximately 768 kB/s. Synchronization and control of the spatiallydistributed audio reproduction devices require a small number ofadditional data to be transmitted such that the final data rate isapproximately 840 kB/s. The resulting symbol rate of 210 kS/s can beaccommodated with a roll-off factor of 19% within a 250-kHz-widechannel. As a result, eight HF carriers, each with two audio channels,are available within the 2-MHz-wide segment between 863 MHz and 865 MHz.

A fully expanded system having six-channel sound typically requiresthree of the eight HF channels, with the result that two of thesesystems can be operated in parallel within a house without interferingwith each other. However, often the center and sub-loudspeaker areconnected directly by wire to the playback device, with the result thatonly two HF channels are needed. In addition, the system provides fordynamic assignment of the channels, with the result that a singlecarrier is used for one stereo signal, even when more than two speakersare operated. The fundamental consideration is that two antennas be setup sufficiently separated from each other on at least one side of thetransmission path, with a single antenna on the opposite side, to formtwo mutually independent transmission links. This fundamentalconsideration is also valid in the case in which the two antennas arelocated on the transmitter side. Where a backward channel is lacking,the transmitter typically cannot choose between the two antennas sinceit does not have any information about the respective receptionconditions. As a result, the useful signal is transmitted twice toobtain the diversity gain, without simultaneously causing a mutualdegradation of the two signals. A solution is the above-mentioned spacetime coding method, whose space-time block codes (STBC) or space timetrellis codes (STTC) meet this requirement.

The table of FIG. 5 illustrates the STBC method of coding andtransmitting a data sequence D_(o) on the line 70 (FIG. 4) with data A,B, C, D. The first line labeled “clock” indicates the successive clocktimes T₁, T₂, T₃, T₄ for the original data sequence D_(o) andtransmission of the symbols. The original data sequence D_(o) with dataA, B, C, D is in the second, line. The third and fourth lines indicate afirst data sequence for the data D₁ on the line 72 obtained byconversion with the data A, −B*, C, −D*, and a second data sequence forthe data D₂ on the line 74 with the data B, A*, D, C*. The third andfourth lines represent the symbol sequences that are transmitted usingquadrature signals by the two antennas 82, 84. The asterisk *illustrated as part of various data values indicates the complexconjugate of that particular data value. The fifth line indicates theeven and odd times for the times T₁ through T₄. The sixth line indicatesthe combination of symbols A, B, and C, D to form a first or secondsymbol pair Sy1, Sy2. The data sequences D₁, D₂ may also be combineddifferently, for example, D₁ with A, B*, C, D*, and D₂ with −B, A*, −D,A*, or in other combinations. It suffices that symbols A, B, C, D arecoded differently in the two data sequences and that the appropriateequations are available on the receiving side.

In a first step during time T₁, the two successive symbols A, B aretransmitted in parallel. The antenna 82 transmits the symbol A and theantenna 84 transmits the symbol B. For purposes of differentiation, thetwo successive symbols A, B are identified as a symbol pair, the firstsymbol A being identified as the even symbol, and second symbol B beingidentified as the odd symbol. Subsequently, transposition andtransformation of the two initially transmitted symbols A, B takesplace, with the result that in the second step during time T₂ at theantenna 82 the symbol B is transmitted in the form of the negatedcomplex conjugate as −B*, while the symbol A is transmitted in the formof the complex conjugate as A*. After two steps T₁, T₂, a symbol pair A,B, (i.e., the first symbol pair Sy1) is thus transmitted. During thethird and fourth times T₃, T₄, the second symbol pair Sy2 with symbolsC, D is transmitted in an identical manner. Each symbol is thustransmitted twice. Since, however, there is also a parallel transmissionthrough both of the transmitting antennas 82, 84, the data rate for thedata sequence Dr on the line 100 on the receiver side 90 is identical tothe original data rate of the data sequence D_(r) on the line 70 (FIG.4).

On the receiver side 90, the symbols A, B, or C, D received at the samefrequency and superimposed are separated. Mathematically, thiscorresponds to the solution of a linear equation system with twounknowns A and B:r _(even) =h1·A+h2*B  (Eq. 1)r _(odd) =h2·A*+h1·(−B*)  (Eq. 2)r _(odd) *=h2·A−h1*·B  (Eq. 3)Equation 2 is generated by transformation of Equation 1. Here h1 denotesthe transfer function from the first transmitting antenna 82 to thereceiving antenna 92, while h2 denotes the transfer function from thesecond transmitting antenna 84 to the receiving antenna 92. The receivedsignal value r_(even) at time “even” is comprised of components A and B,and the two transfer functions h1 and h2. The received signal valuer_(odd) at time “odd” is comprised of the components h1, h2, A* and −B*.As long as transfer functions h1 and h2 are known, Equations 1 and 2represent a linear system from which A and B can be determined. If thecomplex conjugate form corresponding to Equation 3 is generated fromboth sides of Equation 2, then the symbols A, B are identical with thesymbols of Equation 1.

The transfer functions h1, h2 are initially unknown. However, theygenerally represent a steady state since the spatial conditions relativeto the data rate change relatively slowly. In addition, if it can beassumed that both transfer functions are initially equal, they then seeka more desirable value by a control action on the receiver side 90. Tothis end, the received signals on the receiver side 90 are multiplied byan inverse transfer function in a linear combination device 108 (seeFIG. 7) which is initially present as an estimated value. The receivedsignals are then adapted by an adaptive algorithm to the actual transferfunctions of the two transmitting antennas 82, 84. Referring also toFIG. 7, the transfer functions h1 and h2, along with their associatedinverse transfer functions h₁ ⁻¹ and h₂ ⁻¹ in the linear combinationdevice 108 together form a linear frequency response. Based on thelinear combination device 108, the symbols A′, B′ received after thetransfer are translated into the quadrature signal plane such that asymbol decision element 110 can determine the associated decided symbolsA″, B″ from these values. If, as a result of transfer changes in thereceived symbols A′, B′, deviations occur relative to the inversetransfer functions h₁ ⁻¹, h₂ ⁻¹ in the linear combination device 108,these deviations are detected essentially as differences by an equationsystem in an arithmetic unit 112. These difference values are thensmoothed by a control loop filter 114 and supplied as correction valuesto the linear combination device 108.

Referring to FIG. 6, a transmitting device 120 includes a signal source122 that supplies an analog audio signal on a line 123 to ananalog-to-digital converter (ADC) 124. The output of the ADC 124 is adata stream D_(o) on a line 126 with a symbol rate determined by adigitization clock t_(s) provided to the ADC 124 on a line 128. Thedigitization clock on the line 128 corresponds to the symbol clock t_(s)generated in a symbol clock generator T, 130, or a multiple thereof. Twodifferent data streams D₁ and D₂ on the lines 132, 134 are generatedfrom the data stream D_(o) on the line 126 in a transmission codingdevice 136. The data streams D₁, D₂ contain the individual symbol pairsA, B, and C, D, with the respective different coding in the quadraturesignal plane as illustrated in FIG. 5. In a high-frequency stage 138,the two data sequences D₁, D₂ on the lines 132, 134 are transferred to adesired high-frequency band by the sine and cosine components of aquadrature carrier signal tr on a line 140 from a high-frequencyoscillator 142, then transmitted separately through antennas 144, 146.For clarity, the required pulse form filter, as well as the filterdevices to avoid interference and alias signals, are not illustrated inFIG. 6 but are readily apparent to one of ordinary skill in the art.

Referring to FIG. 7, a receiving device 150 includes a heterodynereceiver 152 that includes a high-frequency mixer 154 that converts thehigh-frequency signal received through antenna 156 from thehigh-frequency channel f to an intermediate frequency position whichlies approximately in a frequency range of 1 to 2 MHz. The carrier forthe mixer 154 is a high-frequency signal HF on a line 156 from a localoscillator 158. A bandpass filter 160 filters out the desired frequencyband and provides a filtered signal to an analog-to-digital converter(ADC) 162 for digitization. The conversion to an intermediate frequencyuses the ADC 162. In the case of zero-IF conversion or low-IFconversion, there is a splitting into two channels that are inquadrature with each other and also require two analog-to-digitalconverters. Subsequent processing in a decoding device portion 164 maybe implemented digitally and independently of the preceding heterodynereceiver stage 152.

The digitized signal on a line 166 from the ADC 162 is converted by aquadrature mixer 168 and decimation stages (not shown) such that thedata rate of the resulting data stream corresponds to the symbol ratet_(s) or an integral multiple thereof. The quadrature mixer 168 is fedby an oscillator 170 with a signal on a line 172 that comprises sine andcosine components of the down-mixed carrier frequency which also producetwo mixing components at the output of the mixer 168 on a line 174. Ifthe heterodyne receiver circuit 152 is a zero-IF converter or low-IFconverter, then two in-quadrature data paths in the low-frequencyposition are present and the quadrature mixer 168 may be omitted.

The two mixing components on the line 174 comprise digitized signalvalues which may be coupled to the transferred symbols. A switch 176distributes these values synchronously at symbol clock t_(s) on a line177 to two outputs 178, 180 of the switch 176, thereby supplying inputsof a symbol detection device 182.

The signals on the line 174 from the mixer 168 are alternately dividedby the switch 176 between the two inputs of the symbol detection device182, at the output of which the determined symbols can be tapped fromthe received signal. Based on the alternating division and subsequentsolution of the linear equations for the received signals in the linearcombination device 108, the preliminary estimated symbols A′, B′, or C′,D′ of each symbol pair Sy1, Sy2 are available at the outputs of thedevice 108. The decision element 110 generates the decoded symbols A″,B″, or C″, D″ therefrom which are converted by a table 186 intoelectronic data for symbols A, B, C, D for further processing. From theparallel available symbols A, B, or C, D of the symbol pairs, a switch188 alternately controlled at a symbol clock t_(s) on a line 190 from aclock generator 192 regenerates the original data sequence D_(o) on aline 194 with data A, B, C, D. This data stream can then be convertedinto the audio signal for output through the speaker.

During decoding of the symbols, specifically, in the zero-IF or low-IFmethods, a situation may occur in which the carrier is placed in anactive frequency band during mixing. As a result, a large steady-statecomponent is generated in the down-mixed signal. This component maygenerally exceed the operational ranges of the analog-to-digitalconverters. In the process of down-regulating the signal value,resolution may be lost. As a result, a simple control loop may be usedto superimpose a sufficiently large direct component on the analogsignal before digitization until the signal is within the control rangeof the analog-to-digital converters.

The adaptation of the parameters in the linear combination device 108(FIG. 7) is implemented by sending the signals of the two inputs 178,180, and the two outputs A″, C″ and B″, D″ of the symbol decisionelement 182 to one input each of the arithmetic unit 112 for comparison.In the steady-state condition, the received symbols A′, B′, C′, D′, andthe decided symbols A″, B″, C″, D″ are linked by the inverse transferfunctions h₁ ⁻¹, h₂ ⁻² in the linear combination device 108. This isdone up to the point of unavoidable noise components, since the inversetransfer functions compensate the transmission paths. Deviations inlinearity may be determined by the equation systems in the arithmeticunit 112 which generate correction signals that are supplied by thecontrol loop filter 114 to correction inputs of the linear combinationdevice 108.

For the purpose of conversion to the audio signal, however, additionalinformation is typically required, such as the volume, tone, or balancewhich are a function of the specific location of the audio reproductiondevice. The additional control information relates to the location ofthe device within the three-dimensional sound system. That is, theaddress of the device, the data compression method used, information onthe applicable protection measures to secure data during transmission,and synchronization bits to detect the data package beginning and tosynchronize symbol detection. This control information may be inaudiblysuperimposed on the actual audio signal, or transmitted in addition tothis signal. For transmission, a packet format that contains all therequisite control information and addresses in a header may be utilized.The actual data component then contains the data for the audio signal,and also the check bits or empty bits to fill out the individual dataranges.

Since the source data streams may be already digitized, a sampling rateconversion or even recoding with a detour via an analog signal istypically avoided. This however requires the transmission of differentsampling rates such as 44.1 kHz or around 48 kHz, and integral multiplesthereof. The selected data packet structure (a frame) may be 10 ms long.Following a header with synchronization bits and control parameters, twostereo blocks with 2×240 6-bit values each are transmitted at 48 kHz. At44.1 kHz, three stereo blocks with 2×147 6-bit values each aretransmitted. At 44.1 kHz and lower sampling rates, the extraneous bitsin the individual data blocks are filled with a predefined bit sequence.

Referring to FIG. 8, there illustrated are data formats 196, 198 fortransmission of the audio data in the receiver 150 of FIG. 7. Both dataformats represent one data packet 200 each of 10 ms length. The upperdata format 196 is suitable for a source rate of 48 kHz, while the lowerformat 198 is suitable for a source rate of 44.1 kHz. The individualdata blocks for the left and right audio channel L or R alternatelyfollow the header H. A compression may be oriented by pairs to theseblocks such that the decompression can begin on the receiver side eachtime after reception of the first audio block pair L, R. In the upperformat 196, this corresponds to a delay of about 5 ms, while it is 3.3ms for the lower format 198. On the transmitter side, approximately thesame delay value is added, with the result that the specification of lipsynchronicity which requires a delay of less than 20 ms between videoand sound can be met.

Although the present invention has been illustrated and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

1. A method for wireless transmission of audio signals between atransmitting device and a receiving device, the receiving device havingan audio signal reproduction device, the method comprising the steps of:digitizing the audio signals; coding the digitized audio signals;transmitting the coded digitized audio signals as separate data streamsreceiving the transmitted data streams; decoding the received datastreams; outputting the decoded data streams through the audio signalreproduction device; and providing transmitter diversity between thetransmitting device and the receiving device where the transmittingdevice includes two transmitters each having an associated antenna andoperating in the same frequency band, the receiving device having atleast one receiving antenna and at least one receiver for the frequencyband.
 2. The method of claim 1, where the step of digitizing furthercomprises the step of digitizing the audio signals into a data sequence,and where the method further comprises the steps of: converting the datasequence into a first and second data sequence of successive symbolpairs, wherein in the first and second data sequence the symbol pairsthat are related in time include the same symbols; transposing the orderof the symbols within the symbol pairs in the first and second datasequence relative to each other in a time sequence; and implementing achange in coding of quadrature signal components representing the codeddigitized audio signals, wherein the change in coding relates to a signof the symbol.
 3. The method of claim 2, where the change in codingrelates to a transformation of the symbol to its complex conjugatevalue.
 4. The method of claim 1, where each digitized audio signal has aplurality of discrete data points, and where the step of coding furthercomprises the step of assigning a symbol in a quadrature signal plane toeach discrete data point.
 5. The method of claim 1, further comprisingthe step of compressing the digitized audio signals prior to the step oftransmitting and further comprising the step of decompressing thedigitized audio signals after the step of receiving.
 6. The method ofclaim 1, where the separate data streams are transmitted as datapackets, each packet including header information comprising control andauxiliary information, each packet including data corresponding to theaudio signals where each packet includes an even number of data blocksby which data associated with a first and second audio channel arealternately transmitted in blocks.
 7. A system for wireless transmissionof digitized audio signals, comprising: a transmitting device; and areceiving device; where the transmitting device includes a coding devicethat codes the digitized audio signals as data packets; and includes twotransmitters that generate quadrature signals in the same frequency bandwhich are modulated with the data packets and are transmitted by acorresponding antenna for each transmitter, where the antennas arelocated in a spatial relationship for transmitter diversity operation;and where the receiving device includes an audio reproduction device andat least one receiver that receives the transmitted quadrature signals,the receiving device further includes a decoder that decodes thereceived quadrature signals and provides a decoded audio signal to anaudio reproduction device.
 8. The system of claim 7, where the digitizedaudio signals are arranged in a first data sequence, the coding devicegenerates a pair of data sequences from the first data sequence, andwhere the transmitting device include quadrature mixers that convert thepair of data sequences to the same high-frequency band and provide theconverted data sequences to the corresponding antennas for transmission.9. The system of claim 8, where the coding device generates the pair ofdata sequences based on a space-time block code.
 10. The system of claim8, where the pair of data sequences each includes data that representssymbols arranged as successive symbol pairs that are related in time.11. The system of claim 9, where the decoding device decodes the datasequences based on the space-time block code, and where the receivingdevice includes first switch that supplies the received quadraturesignals to a first and second terminal of a linear combination device ata clock period of a symbol rate, a symbol decision element connected totwo outputs of the linear combination device a symbol table connected tothe symbol decision element that supplies a logical level of each of theassociated symbols within the received quadrature signals, and a secondswitch that regenerates the first data sequence from the symbols byalternate switching at the symbol clock period.
 12. A system forwireless transmission and reception of audio signals, comprising atransmitter side and a receiver side, where the transmitter sidecomprises: a source of audio signals that provides the audio signals ina data sequence; an encoder that codes the data sequence into a pair ofdata streams; a pair of transmitters that each transmits a correspondingone of the pair of data streams; and a pair of transmitting antennaseach associated with a corresponding one of the pair of transmitters,the transmitting antennas being located in a spatially-separatedtransmitter diversity relationship with each other, the antennastransmitting the corresponding one of the pair of data streams on afrequency that is that same for each antenna; and where the receiverside comprises a receiving antenna that receives the transmitted pair ofdata streams; a receiver that processes the received data streams; adecoder that recovers the data sequence from the processed data streamsand provides an audio signal in response thereto; and an audioreproduction device that outputs the audio signal from the decoder. 13.The system of claim 12, where the audio signals are in analog format,and where the transmitter side digitizes the audio signals.
 14. Thesystem of claim 12, where the audio signals are in digital format. 15.Ths system of claim 12, where the transmitter side compresses the datasequence, and where the receiver side decompresses the data sequence.16. The system of claim 12, where the encoder codes the data sequenceinto a pair of data streams each comprising symbols.
 17. The system ofclaim 16, where the transmitters transmit the data streams comprisingsymbols using quadrature amplitude modulation, and where the receiverprocesses the received data streams by evaluating the received datastreams at predefined times at which each of the symbols within thetransmitted data streams occupies a defined state in the quadraturesignal plane.
 18. The system of claim 17, where the receiver determinesthe defined state that corresponds to the transmitted symbols bysampling and digitizing the received data streams at least at definedtimes and at a frequency that is lower than the frequency that the datastreams are transmitted.
 19. The system of claim 16, where the encodercodes the data sequence using space-time block codes, and where thereceiver processes the received data streams using space-time blockcodes.
 20. The system of claim 12, where the transmitters modulate thepair of data sequences onto a high-frequency carrier signal, and wherethe receiver converts the received data streams to a frequency that islower that that of the high-frequency carrier signal.